If they just want a small wedding (say 50 people) then I say have that small wedding, but not on the cruise ship. Logistically it seems difficult and etiquettely it seems a bit dodgy for people to come all that way for a light reception, then be kicked off the ship unless they were willing and able to pay a lot more to cruise with the HC on their honeymoon. And to me that option seems a bit weird, too, honestly--it's one thing, maybe, if your parents or siblings take the same cruise as you for your honeymoon, but if it's just opened up to any guest who can pay, you might end up with annoying Great-Aunt Matilda or Cousin Floyd, who have always wanted to try cruising and think this is a great opportunity.
What I would suggest is a small wedding near the cruise ship, but easily accessible on land. Then you can go all-out with the reception and make it really nice for guests, especially if they still have to travel a long way. Then, the HC jaunts off for their honeymoon cruise. In a scenario like this, I would not have another, larger reception when they return. This would be just a small wedding, not something extreme (like getting married while climbing Mt. Everest), so I think you just get this one shot at the celebration.
I'm not familiar with cruising terminology... Does "five hours from port" mean that guests have to travel, by water, five hours to reach the ship? And then five hours back? Because I think that would be a non-starter right there--it would be like arranging for a cruise just to get to the wedding location, where you can't even feasibly stay the night, so ten hours of travel time in one day with a brief wedding in between.